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A new study, published online at The Lancet shows that the bivalent polio vaccine, which is currently in use in India and Nigeria and offers protection against two of the polio virus strains, type 1 & 3, triggers a stronger immune response than the existing trivalent vaccine and similar immune response to the monovalent vaccines.

Study Summary – This was a double-blind, randomized, controlled study which enrolled 830 babies in India, between August and December 2008. The researchers compared various oral polio vaccines’ efficacy in inducing an immune response, measured by the number of antibodies created after the doses were received (seroconversion). The total amount of antibodies was measured and compared after the first dose, and also after a second dose.

The babies were set up in 5 groups; it is not clear from the summary but it appears that each group would have received one of the following vaccines:

monovalent type 1

monovalent type 2

monovalent type 3

bivalent 1 & 3

trivalent.

Although it is possible that some group may have been given a combination of monovalent vaccines; I am not sure. What does mono, bi, trivalent mean? It means this: the monovalent vaccines protect against one type only of the virus that causes polio. For example, monovalent type 1 protects against the Type 1 of the polio virus. Bivalent vaccines protect against two types at the same time, and trivalent vaccine protects against 3 types at the same time.

Immune response, or seroconversion, was measured after the first dose, and after the second dose of the vaccines. The responses were compared for the various vaccines.

Results – The results were as such:

Seroconversion after Dose 1

Type 1 Virus

Monovalent – 20%

Bivalent – 20%

Trivalent – 15%

Type 2 Virus

Monovalent – 21%

Bivalent – N/A

Trivalent – 25%

Type 3 Virus

Monovalent – 12%

Bivalent – 7%

Trivalent – 4%

Seroconversion after Dose 2 (cumulative)

Type 1 Virus

Monovalent – 90%

Bivalent – 86%

Trivalent – 63%

Type 2 Virus

Monovalent – 90%

Bivalent – N/A

Trivalent – 91%

Type 3 Virus

Monovalent – 84%

Bivalent – 74%

Trivalent – 52%

The vaccines were well tolerated. 19 serious adverse events occurred, including one death; however, these events were not attributed to the trial interventions.

Conclusion – This study shows statistically significant differences between the bivalent and the trivalent vaccine, differences that become clearer after the second dose, at which point the bivalent vaccine outscored the trivalent one by more than 20% points for both polio viruses Type 1 and 2. There are no statistically significant differences between the bivalent and monovalent vaccines.

The conclusion following the results of this study is that, in this study the bivalent vaccine worked better than the trivalent vaccine in inducing an immune response, in infants. Further, the bivalent vaccine rates of seroconversion was just as good as the monovalent vaccines ones.

It is important to keep in mind though, that this study was only measuring the immune response, and does not draw any conclusions about the reduction of polio infections, hospitalizations or death rates. If that was the goal of the study, a proper placebo control would be absolutely necessary, but given that the purpose of the study was to compare efficacy of seroconversion rates as compared to the trivalent/monovalent vaccines, the use of a placebo is not necessary, since it is logical to assume that any placebo effects would similarly affect all groups of participants.

As such this study, in and off itself, does not lead to any conclusions about the bivalent vaccine’s efficacy in preventing polio infections, hospitalizations and deaths. We may extrapolate given it’s seroconversion rates, and what it is known about the monovalent/trivalent vaccines effects on polio infections/hospitalizations/death rates, but that would be just that, an extrapolation. The only question this study directly answers is: How does the bivalent polio vaccine compare to the monovalent and trivalent polio vaccines in inducing an immune response?

The authors concluded as such:

The findings show the superiority of bOPV compared with tOPV, and the non-inferiority of bOPV compared with mOPV1 and mOPV3.